Photoconductive composition containing a tricyanopyrene, article and process of use

ABSTRACT

Photoconductive composition comprising an insulating polymeric matrix and a compound of the formula ##STR1## wherein R is --H or --CN; 
     R&#39;, r&#34;, r&#39;&#34; and R iv  are independently selected from an aliphatic hydrocarbon radical having from about 1-10 carbon atoms; phenyl; or substituted phenyl wherein said substituents are capable of releasing electrons to relatively electron deficient centers within the compound; amino; diarylamino; dialkylamino or alkoxy; n can range from 0 up to the potential number of positions of substitution on the aromatic ring system. 
     These compositions have good spectral response in the visible region of the electromagnetic spectrum and are suitable for use in electrostatographic imaging members and methods.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation in part of copending application Ser.No. 454,487, filed Mar. 25, 1974 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a composition, an article and a method. Morespecifically, the compositions embraced within the scope of thisinvention are highly efficient photogenerator materials and are thussuitable for use in electrophotographic imaging members and methods.

2. Description of the Prior Art

The formation and development of images on the imaging surfaces ofphotoconductive materials by electrostatic means is well-known. The bestknown of the commercial processes, more commonly known as xerography,involves forming a latent electrostatic image on the imaging surface ofan imaging member by first uniformly electrostatically charging thesurface of the imaging layer in the dark and then exposing thiselectrostatically charged surface to a light and shadow image. The lightstruck areas of the imaging layer are thus rendered relativelyconductive and the electrostatic charge selectively dissipated in theseirradiated areas. After the photoconductor is exposed, the latentelectrostatic image on this image bearing surface is rendered visible bydevelopment with a finely divided colored electroscopic material, knownin the art as "toner". This toner will be principally attracted to thoseareas on the image bearing surface which retain the electrostatic chargeand thus form a visible powder image.

The developed image can then be read or permanently affixed to thephotoconductor where the imaging layer is not to be reused. This latterpractice is usually followed with respect to the binder-typephotoconductive films (e.g. zinc oxide/insulating resin binder) wherethe photoconductive imaging layer is also an integral part of thefinished copy, U.S. Pat. Nos. 3,121,006 and 3,121,007.

In so-called "plain paper" copying systems, the latent image can bedeveloped on the imaging surface of a reusable photoconductor ortransferred to another surface, such as a sheet of paper, and thereafterdeveloped. When the latent image is developed on the imaging surface ora reusable photoconductor, it is subsequently transferred to anothersubstrate and then permanently affixed thereto. Any one of a variety ofwell-known techniques can be used to permanently affix the toner imageto the copy sheet, including overcoating with transparent films, andsolvent or thermal fusion of the toner particles to the supportivesubstrate.

In the above "plain paper" copying systems, the materials used in thephotoconductive layer should preferably be capable of rapid switchingfrom insulating to conductive to insulating state in order to permitcyclic use of the imaging surface. The failure of a material to returnto its relatively insulating state prior to the succeedingcharging/imaging sequence will result in an increase in the rate of darkdecay of the photoconductor. The phenomenon, commonly referred to in theart as "fatigue" has in the past been avoided by the selection ofphotoconductive materials possessing rapid switching capacity. Typicalof the material suitable for use in such a rapidly cycling imagingsystem include anthracene, sulfur, selenium and mixtures thereof (U.S.Pat. No. 2,297,691); selenium being preferred because of its superiorphotosensitivity

In addition to anthracene, other organic photoconductive materials, mostnotably, poly(N-vinylcarbazole), have been the focus of increasinginterest in electrophotography, U.S. Pat. No. 3,037,861. Until recently,neither of these organic materials have received serious considerationas an alternative to such inorganic photoconductors as selenium, due tofabrication difficulties and/or to a relative lack of speed andphotosensitivity within the visible band of the electromagneticspectrum. The recent discovery that high loadings of2,4,7-trinitro-9-fluorenone in polyvinylcarbazoles dramatically improvesthe photoresponsiveness of these polymers has led to a resurgence ininterest in organic photoconductive materials, U.S. Pat. No. 3,484,237.Unfortunately, the inclusion of high loadings of such activators can andusually does result in phase separation of the various materials withinsuch as composition. Thus, there will occur within these compositionsregions having an excess of activator, regions deficient in activatorand regions having the proper stoichiometric relation of activator tophotoconductor. The maximum amount of activator that may be added tomost polymeric photoconductive materials without occasioning such phaseseparation generally will not exceed in excess of about 6 to about 8weight percent.

One method suggested for avoiding the problems inherent in the use ofsuch activators in conjunction with polymeric photoconductors, is thedirect incorporation of the activators into the polymeric backbone ofthe photoconductor, U.S. Pat. No. 3,418,116. In this patent is disclosedthe copolymerization of a vinyl monomer having an aromatic and/orheterocyclic substituent capable of an electron donor function with avinyl monomer having an aromatic and/or heterocyclic substituent capableof an electronic acceptor function. The spatial constraint placed uponthese centers of differing electron density favors their charge transferinteraction upon the photoexcitation of such a composition. Theseso-called "intramolecular" charge transfer complexes, more accuratelydesignated "intrachain" charge transfer complexes, are believed tofunction substantially the same as charge transfer complexes formedbetween small activator molecules and a photoconductive polymer. Thefact that the electron donor function and an electron acceptor functionare on a common polymeric backbone does not apparently change the π - πcharge transfer interaction, but merely increases the probability of itoccurring. Unfortunately, the preparation of such polymers from vinylmonomers having electron donor centers and vinyl monomers havingelectron acceptor centers is often beset with difficulty.

The preparation of non-polymeric photoconductive tricyanovinylcompounds, wherein an electron rich center and an electron deficientcenter are contained within a common molecule, is disclosed in U.S. Pat.No. 3,721,552 (corresponding Australian patent application Ser. No.36,760/68, published Oct. 10, 1969). Patentee discloses the preparationof photoconductive "binder " layers by dispersing from about 10 to about90 parts by weight of his novel tricyanovinyl compounds in about 90 toabout 10 parts by weight resin binder. The binder resins which can beused in preparation of the photoconductive insulating layer must have anelectrical volume resistivity in excess of 10⁸ ohm - cm. Virtually anyof the binders traditionally employed in preparation ofelectrophotographic imaging members are reportedly suitable in thepreparation of these binder layers. Insofar as the preferred weightratio of photoconductive particles to binder resin is 1:1, it isapparent that Patentee does not appreciate that sufficiently lowerloadings of such compounds in a charge transport matrix can produceresults equivalent to his preferred composition. By minimizing theamount of photoconductive compound needed to achieve satisfactoryphotoresponse, the inherent physical properties of the film formingbinder resin are preserved (e.g. flexibility, adhesion, and free surfaceenergy).

It is the principal object of this invention to provide a novel class ofphotogenerator compounds which are suitable for use in photoconductivecompositons.

It is another object of this invention to provide a photogeneratorcompound having a high extinction coefficient.

It is yet another object of this invention to provide a photogeneratorcompound wherein charge transfer interaction between a donor andacceptor site occur independent of the relative concentration of thephotogenerator compounds in the resin.

It is yet a further object of this invention to provide aphotoconductive composition having broad spectral response in thevisible region of the electromagnetic spectrum.

Further objects of this invention include providing imaging memberswherein the imaging layer is prepared from the above composition and theuse of said imaging members in an imaging method.

SUMMARY OF THE INVENTION

The above and related objects are achieved by providing aphotoconductive composition comprising an insulating polymeric matrixand a compound of the formula ##STR2## wherein R is --H or --CN;

R', r", r'" and R^(iv) are independently selected from an aliphatichydrocarbon radical having from about 1 - 10 carbon atoms; phenyl orsubstituted phenyl wherein said substituents are capable of releasingelectrons to relatively electron deficient centers within the compound;amino; diarylamino; dialkylamino or alkoxy; n can range from 0 up to thepotential number of positions of substitution on the aromatic ringsystem.

In the preferred embodiments of this invention, the above polymericmatrix is also capable of rapid and efficient transport of chargedcarriers generated during photoexcitation of the above compound. In suchpreferred embodiments of this invention, the concentration ofphotogenerator compound is generally less than 50 weight percent.

DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS

The composition of this invention can be prepared by combining one ormore of the hereinbefore described photogenerator compounds and thevarious other materials comprising an insulating polymeric matrix in acommon solvent and casting or coating the resulting solution on anappropriate (preferrably conductive) substrate. The relativeconcentration of photogenerator compound to matrix materials in suchcompositions will vary with the transport capabilities of the polymericmatrix. The insulating polymer matrices suitable for use in thisinvention can be either "electronically active" or "electronicallyinert". The classification of the matrix as active or inert isdetermined by the relative ability of the matrix, when used inconjunction with the photogenerator, to transport charge. Thosematerials which are capable of efficient transport of at least onespecies of photogenerated charge carrier are considered electronicallyactive and the insulating polymeric matrix classified as an "activematrix". Conversely, those materials which do not exhibit transport ofat least one species of photogenerated charge carrier are regarded aselectronically inert and the insulating polymeric matrix classified asan "inert matrix". Electronic activity (or inertness) of a matrix is,therefore, intended to be descriptive of two separate events, both ofwhich must occur; the capacity (or incapacity) of the matrix to permitinjection of photogenerated charge carriers into its bulk and thecapacity (or incapacity) of the matrix to transport such injected chargecarriers through its bulk without trapping.

Virtually, any of the polymeric binders disclosed in the prior art canbe used in combination with the photogenerator compounds disclosedherein. Representative of the electronically inert binders suitable foruse in the compositions of the invention include epoxy resins,poly(vinylchloride), poly(vinylacetates), poly(styrene),poly(butadiene), poly(methacrylates), poly(acrylics),poly(acrylonitriles), silicone resins, chlorinated elastomers, phenoxyresins, phenolic resins, epoxy/phenolic copolymers,epoxy/urea/formaldehyde terpolymers, epoxy/melamine/formaldehyde resins,poly(carbonates), poly(urethanes), poly(amides) saturated poly(esters)copolymers and blends thereof. Electronically active polymers which canbe used as the matrix for the photogenerator compound includepoly(N-vinylcarbazole), poly(2-vinylcarbazole), poly(3-vinylcarbazole),poly(vinylpyrene), poly(vinylnaphthalene), poly(2-vinylanthracene) andpoly(9-vinylanthracene). Electronically active matrices can also beformed by combination of one or more of the above electronically inertpolymers with one or more of the above electronically active polymers.The method of combination of such electronically distinct polymers caninclude copolymeriation (random, graft, block, etc.), formation of aninterpenetrating polymer network and polymer blending. Alternatively, anelectronically inert polymer matrix can be rendered an efficienttransporter of charge carriers by the incorporation within a film ofsuch materials so called "small molecules" capable of an efficientcarrier transport. The term, "small molecules", is inclusive of singlemolecules and low molecular weight polymers. These small molecules canbe added to the casting or coating solution during formation of thepolymeric matrix or can be subsequently introduced into the matrix byswelling of the polymeric materials of the matrix with a solutioncontaining the small molecule compounds. Upon evaporation of the liquidphase of the solution, the small molecules will remain entrapped withinthe polymeric matrix thus enhancing charge carrier transport propertiesof this insulating film. These small molecules can also be added toactive polymeric matrices in order to enhance the transport of chargecarriers not readily transported by the electronically active polymer.For example, Lewis Acid can be added to a photoconductive polymer suchas poly(N-vinylcarbazole) in order to improve electron transport.Representative of small molecule additives, which can be added to eitheran electronically active or inert polymer matrix to facilitate hole (+)transport include pyrene, anthracene, carbazole, triphenylamine,naphthalene, julolidine, indole and perylene. Small molecule additive,which can be incorporated into either an electronically active or inertpolymer matrix to facilitate electron (-) transport include anthracene,fluorenone 9-dicyanomethylene-fluorene, the nitro derivatives offluorenone, the nitro derivatives of 9-dicyanomethylene-fluorene andchoranil. Both hole and electron small molecule transport materials canbe used in combination with one another in inert polymers. A number ofthe above small molecules are known to form charge transfer complexeswith both the inert and active polymer systems and some absorption bythe matrix complex is permitted provided that the absorptivity of theresulting change transfer complex does not compete with thephotogenerator compound to the extend that the absorption band of thecomposition is dominated by the absorption band of the complex. It isalso understood that the absorptivity of the charge transfer complexmust not be capable of shielding the photogenerator compound fromincident radiation.

The photogenerator compounds of this invention, which satisfy thepreviously set forth structural formula, are part of a unique class ofcompounds that have both an electron withdrawing group and an electronreleasing group connected to one another through a spatiallyconstraining linkage thereby insuring that during photoexcitation of thepolymeric matrix containing such compounds, the electronic transitionmoment from ground to excited state and flow of charge between saidgroups are collinear. Thus, the generation of charge carriers uponphotoexcitation said compounds is highly efficient even at very lowconcentrations (<˜6 weight percent). Of course, at such low loadings thepolymeric matrix must be electronically active in order to transport thecarriers generated during exposure to electromagnetic radiation. In thepreferred embodiments of this invention the concentration ofphotogenerator compound in an electronically active matrix can rangefrom as low as about 0.1 to about 6 weight percent and yet providesatisfactory electrophotographic response. At such low concentrationsthe photoconductive composition can be described as a solid solution,i.e. a single phase composition formed between the photogeneratorcompound and the polymeric materials of the matrix in which homogeneityis not due to compound formation, Van Norstrand's ScientificEncyclopedia, 4th Ed., D. Van Norstrand Company Inc., p. 1651 (1968). Ofcourse, where small molecules are added to polymeric materials toenhance transport of one or both species of charge carriers, thehomogeneity of the composition may be altered somewhat.

At concentrations in excess of 6 weight percent (up to about a maximumof about 99.9 weight percent) the tendency for crystallization of thephotogenerator compound within the matrix will increase. As the extentof crystallization increases, the physical properties of the polymermatrix will become impaired and the ability of the photoconductivecomposition to hold charge will also shown progressive decline.

As indicated previously, the compositions of this invention can bereadily prepared by simply combining the photogenerator compound and thefilm forming insulating polymer in the proper relative proportions in acommon solvent and thereafter casting or coating the resulting solutionon an appropriate substrate. The amount of material coated on suchsubstrates should be sufficient to provide a dry film having a thicknessin the range of from about 0.1 to about 200 microns; the precisethickness being determined by the end use of said member. Any of thesubstrates traditionally used in preparation of electrophotographicimaging members can be coated with the above solution. Typical ofsubstrates which are suitable in this regard include aluminum, chromium,nickel, brass, metallized plastic film, metal coated plastic film (e.g.aluminized Mylar) and conductive glass (e.g. tin oxide coated glass --NESA glass).

Upon preparation of an electrophotographic imaging member from thematerials described above, said member can be used in standardelectrophotographic imaging methods by simply sensitizing the surface ofthe photoconductive insulating layer of said member followed by exposureof the sensitized surface to a light and shadow image pattern. Where thephotogenerator compound is dispersed in an electronically active polymermatrix, the wavelength of activating electromagnetic radiation shouldpreferably be within the wavelength of substantial spectral response ofthe photogenerator compound and outside the range of substantialspectral response of the electronically active polymer matrix. Uponformation of latent electrostatic image on said member, the image may betransferred to another substrate or developed directly on said imaginglayer and thereafter transferred. Where one or more of suchphotogenerator compounds are incorporated within an electronicallyactive polymer or an electronically active polymer containing a smallmolecule compound, the absorption spectrums of the composition arecharacteristic of the individual components of the composition, thus,indicating no discernable interaction between the photogeneratorcompound and the matrix.

In those compositions where the relative concentration of photogeneratorcompound adversely alters the charge storage capacity of thecomposition, films prepared from such compositions can be overcoatedwith an insulating (electronically "inert") polymer film. The dielectricthickness of this overcoating must be sufficient to support at leastsome, if not the entire, sensitizing charge. Such overcoated imagingmembers are suitable for use in induction imaging systems of the typedisclosed in U.S. Pat. No. 3,324,019 (to Hall); U.S. Pat. No. 3,676,117(to Kinoshita) and U.S. Pat. No. 3,653,064 (to Inoue) --all of which arehereby incorporated by reference in their entirety. In the imagingsystem described by Inoue, the insulating overcoating is subjected touniform corona charging in the light (the polarity of the charge beingimmaterial). The sensitized imaging member is now exposed to imageinformation simultaneous with corona charging to opposite polarity. Theimaged member is thereafter exposed to blanket illumination and a latentimage thus produced developed with charged electroscopic toner particlesand thereafter transferred to a receiving sheet.

The Examples which follow further define, describe and illustratepreparation and use of the compositions of this invention. Methods ofpreparation and evaluation of said compositions are standard or ashereinbefore described. Parts and percentages appearing in such Examplesare by weight or otherwise indicated.

EXAMPLE I Preparation of 1-dicyanovinylpyrene

About 100 grams (0.434 moles) pyrene-1-carboxaldehyde, about 28.7 grams(0.434 moles) malononitrile, about 10.4 milliliters acetic acid, about3.48 grams ammonium acetate and about 400 milliliters benzene are heatedto boiling under reflux conditions for 30 minutes. The entire contentsof the flask crystallized, forming a deep orange red product which issoluble in -tetrahydrofuran, moderately soluble in ethanol and verysoluble in chlorobenzene. This product is recrystallized from a mixtureof chlorobenzene and ethanol (30:70). Yield: 104.4 grams m. p 248°-250°C.

About 5 parts by weight 1-dicyanovinylpyrene and 95 parts by weightpoly(N-vinylcarbazole) are dissolved in tetrahydrofuran and draw coatedon an aluminized Mylar substrate. The coated substrate is nowtransferred to a vacuum oven and allowed to remain there overnight.Sufficient solution is transferred to the substrate to provide a polymercoating having a dry film thickness of about 35 microns. The polymerbecomes intensely colored upon admixture with the photogeneratorcompound, however, remains substantially homogeneous. Thephotoconductive insulating layer thus produced is sensitized by coronacharging to a negative potential of about 600 volts. This sensitizedsurface is exposed through a quartz glass transparency with a 100 watttungsten lamp from a distance of 50 centimeters for an intervalsufficient to selectively discharge the exposed surface of thephotoconductive insulating layer an thereby form a latent electrostaticimage. This latent electrostatic image is developed with positivelycharged toner particles and the toner image thereafter transferred to asheet of untreated paper. Toner residues remaining on the surface of thefilm are removed by wiping with a soft cotton cloth. Prior toresensitization, the photoconductive insulating layer is subjected toblanket exposure with ultraviolet light simultaneous with positivecorona charging. The copying cycle is the repeated. Copy quality remainsgood and is reproducible.

What is claimed is:
 1. A photoconductive composition comprising a solidsolution of at least one photogenerator compound of the formula ##STR3##wherein R is -- H or --CN;R', r", r'" and R^(iv) are independentlyselected from an aliphatic hydrocarbon radical having from about 1 - 10carbon atoms; phenyl; or substituted phenyl wherein said substituentsare capable of releasing electrons to relatively electron deficientcenters within the compound; amino; diarylamino; dialkylamino or alkoxy;n can range from 0 up to the potential number of positions ofsubstitution on the aromatic ring systemand an insulating polymericmatrix, said polymeric matrix being capable of rapid and efficienttransport charge carriers of at least one polarity.
 2. The compositionof claim 1, wherein the photogenerator compound is 1-dicyanovinylpyrene.3. The composition of claim 1, wherein the photogenerator compound is1-tricyanovinylpyrene.
 4. A composition comprising from about 0.1 toabout 99.9 weight percent of at least one photogenerator compound of theformula ##STR4## wherein R is --H or --CN;R', r", r'"and R^(iv) areindependently selected from an aliphatic hydrocarbon radical having fromabout 1 - 10 carbon atoms; phenyl; or substituted phenyl wherein saidsubstituents are capable of releasing electrons to relatively electrondeficient centers within the compound; amino; diarylamino; dialkylaminoor alkoxy; n can range from 0 up to the potential number of positions ofsubstitution on the aromatic ring systemin an insulating polymericmatrix, the minimum concentration of photogenerator compound relative topolymeric matrix being sufficient to render the compositionphotoconductive.
 5. The composition of claim 4, wherein thephotogenerator compound is 1-dicyanovinylpyrene.
 6. The composition ofclaim 4, wherein the photogenerator compound is 1-tricyanovinylpyrene.7. An electrophotographic imaging member comprising a conductivesubstrate and a photoconductive insulating layer operatively disposed inrelation thereto, said photoconductive insulating layer comprising acomposition containing from about 0.1 to about 99.9 weight percent of atleast one photogenerator compound of the formula ##STR5## wherein R is--H or --CN;R', r", r'" and R^(iv) are independently selected from analiphatic hydrocarbon radical having from about 1 - 10 carbon atoms;phenyl; or substituted phenyl wherein said substituents are capable ofreleasing electrons to relatively electron deficient centers within thecompound; amino; diarylamino; dialkylamino or alkoxy; n can range from 0up to the potential number of positions of substitution on the aromaticring systemin an insulating polymeric matrix, the minimum concentrationof photogenerator compound relative to polymeric matrix being sufficientto render the composition photoconductive.
 8. The imaging member ofclaim 7, wherein the photogenerator compound is 1-dicyanovinylpyrene. 9.The imaging member of claim 7, wherein the photogenerator compound is1-tricyanovinylpyrene.
 10. An electrostatographic imaging processcomprising:a. providing an electrophotographic imaging member having aconductive substrate and a photoconductive insulating layer operativelydisposed in relation thereto, said photoconductive insulating layercomprising a composition containing from about 0.1 to about 99.9 weightpercent of at least one photogenerator compound of the formula ##STR6##wherein R is --H or --CN;R', r", r'" and R^(iv) are independentlyselected from an aliphatic hydrocarbon radical having from about 1 - 10carbon atoms; phenyl; or substituted phenyl wherein said substituentsare capable of releasing electrons to relatively electron deficientcenters within the compound; amino; diarylamino; dialkylamino or alkoxy;n can range from 0 up to the potential number of positions ofsubstitution on the aromatic ring system in an insulating polymericmatrix, the minimum concentration of photogenerator compound relative topolymeric matrix being sufficient to render the compositionphotoconductive; and b. forming a latent electrostatic image on saidmember.
 11. The imaging process of claim 10, wherein the photogeneratorcompound is 1-dicyanovinylpyrene.
 12. The imaging process of claim 10,wherein the photogenerator compound is 1-tricyanovinylpyrene.